1. Field of the Invention
The present invention relates to an amplifier/oscillator circuit and more particularly, to an amplifier/oscillator circuit including a common-emitter amplifier and a differential amplifier interconnected with each other without using any coupling capacitor, which is applicable to a reference oscillator for a Phase-Locked Loop (PLL) circuit.
2. Description of the Prior Art
Conventionally, to constitute an oscillator circuit using a crystal-controlled oscillator (i.e., a crystal oscillator) and an amplifier, it has been popular to connect the crystal oscillator to the amplifier through a coupling capacitor. This is because the bias voltage of the crystal oscillator at the output terminal(s) is different from the bias voltage of the amplifier at the input terminal(s). Due to the behavior of this coupling capacitor, the ac (or, signal) component of the output signal of the crystal oscillator is able to be transmitted to the amplifier while preventing the bad effects caused by the different bias voltages.
Various oscillator circuits using the coupling capacitor have been well known. Some examples are disclosed in (a) a book written by Tamotsu Inaba, entitled "Mastering Oscillator Circuits", FIGS. 7-16 on pp. 153, published in Sep. 20, 1988 from Nihon Hoso Kyokai (NHK), (b) FIG. 2 of the Japanese Non-Examined Utility-Model Publication No. 2-36213 published in 1990, and (c) FIGS. 1 and 2 of the Japanese Non-Examined Patent Publication No. 8-340216 published in 1996.
A typical example of the conventional oscillator circuits is shown in FIG. 1. This oscillator circuit 55 is comprised of a crystal oscillator 51 located at a first stage, a first differential amplifier 53 located at a second stage, and a second differential amplifier 54 located at a third stage.
The crystal oscillator 51 includes a common-emitter amplifier 51a, a crystal X.sub.tal, and two capacitors C.sub.51 and C.sub.52.
The common-emitter amplifier 51a has an npn-type bipolar transistor Q51, a feedback resistor R.sub.f5, and a load resistor R51. An emitter of the transistor Q51 is directly connected to the ground. A base of the transistor Q51 is directly connected to an input terminal T51. A collector of the transistor Q51 is connected through the load resistor R51 to a power supply line applied with a power supply voltage V.sub.CC. The base and collector of the transistor Q51 are coupled together through the feedback resistor R.sub.f5. The collector of the transistor Q51 is directly connected to another input terminal T52.
A terminal of the crystal X.sub.tal is connected to the ground through the capacitor C.sub.51. Another terminal of the crystal X.sub.tal is connected to the ground through the capacitor C.sub.52. These two terminals of the crystal X.sub.tal are connected to the input terminals T51 and T52, respectively. Thus, the two terminals of the crystal X.sub.tal are connected to the base and collector of the transistor Q51 through the input terminals T51 and T52, respectively.
The first differential amplifier 53, which serves as a buffer amplifier, has a pair of emitter-coupled npn-type bipolar transistors Q52 and Q53, a constant current sink 61 sinking a constant current I.sub.O51 for driving the pair, two base resistors R52 and R55, and two load resistors R53 and R54. The coupled emitters of the transistors Q52 and Q53 are connected to the ground through the constant current sink 61. The collectors of the transistors Q52 and Q53 are connected to the power supply line of V.sub.CC through the load resistors R53 and R54, respectively. Bases of the transistors Q52 and Q53 are connected to the power supply line of V.sub.CC through the base resistors R52 and R55, respectively.
The bases of the transistors Q52 and Q53 serve as input terminals of the first differential amplifier 53. The collectors of the transistors Q52 and Q53 serve as output terminals of the amplifier 53.
The base of the transistor Q52 (i.e., one of the input terminals) of the first differential amplifier 53 is further connected to the base of the transistor Q51 (i.e., the output terminal of the oscillator 51) of the common-emitter amplifier 51a through a coupling capacitor C.sub.C51.
The second differential amplifier 54 has a pair of emitter-coupled npn-type bipolar transistors Q54 and Q55, a constant current sink 62 sinking a constant current I.sub.O52 for driving the pair, and two load resistors R56 and R57. The coupled emitters of the transistors Q54 and Q55 are connected to the ground through the constant current sink 62. The collectors of the transistors Q54 and Q55 are connected to the power supply line of V.sub.CC through the load resistors R56 and R57, respectively. The collectors of the transistors Q54 and Q55 are further connected to output terminals T53 and T54, respectively. Bases of the transistors Q54 and Q55 are connected to the collectors of the transistors Q53 and Q52, respectively.
The bases of the transistors Q54 and Q55 serve as input terminals of the second differential amplifier 54. The collectors of the transistors Q54 and Q55 serve as output terminals of the amplifier 54 or the oscillator circuit 55.
The first and second differential amplifiers 53 and 54 are formed on an IC (not shown) having the input terminals T51 and T52 and the output terminals T53 and T54. The crystal X.sub.tal and the two capacitors C.sub.51 and C.sub.52 are produced as a unit, and located outside the IC. The unit including the crystal X.sub.tal and the capacitors C.sub.51 and C.sub.52 is connected to the IC through the input terminals T51 and T52.
The conventional oscillator circuit 55 shown in FIG. 1 operates in the following way.
The crystal oscillator 51 is an oscillator with the negative-feedback configuration using the feedback resistor R.sub.f5. An output signal of the crystal oscillator 51, which has a fixed frequency defined by the crystal X.sub.tal, is produced at the base of the transistor Q51.
The output signal of the crystal oscillator 51 is applied to the base of the transistor Q52 of the first differential amplifier 53 through the coupling capacitor C.sub.C51 while preventing the dc bias applied to the crystal oscillator 51 from being transmitted to the first differential amplifier 53. Two output signals of the first differential amplifier 53, which are proportional to the difference between the applied output signal of the crystal oscillator 51 and the voltage applied to the base of the transistor Q53, are produced at the collectors of the transistors Q52 and Q53.
The two output signals of the first differential amplifier 53 are directly applied to the bases of the transistors Q55 and Q54 of the second differential amplifier 54, respectively. Two output signals of the second differential amplifier 54, which are proportional to the difference between the two output signals of the first differential amplifier 53, are produced at the collectors of the transistors Q54 and Q55, i.e., the output terminals T53 and T54.
Thus, the two output signals of the conventional oscillator circuit 55 shown in FIG. 1, which have the fixed frequency defined by the crystal X.sub.tal, are derived at the output terminals T53 and T54.
Another example of the conventional oscillator circuits is shown in FIG. 2. This oscillator circuit 55' has the same configuration as that of the oscillator circuit 55 shown in FIG. 1 except that the common-emitter amplifier 51a of the crystal oscillator 51 is replaced with a common-emitter amplifier 51a'. Therefore, the explanation about the same configuration and operation are omitted here by attaching the same reference symbols as those in the circuit 55 to the same elements in FIG. 2 for the sake of simplification of description.
In the common-emitter amplifier 51a', as clearly seen from FIG. 2, the collector (rather than the base) of the bipolar transistor Q51 is connected to the corresponding terminal of the coupling capacitor C.sub.C51, which is unlike the circuit 55 in FIG. 1.
Still another example of the conventional oscillator circuits is shown in FIG. 3. This oscillator circuit 75 has the same configuration as that of the circuit 55 shown in FIG. 1 except that the first differential amplifier 53 is replaced with a differential amplifier 73.
In the differential amplifier 73, as clearly seen from FIG. 3, the combination of a base resistor R71 and a constant current sink 71 sinking a constant current I.sub.O71 is provided between the coupling capacitor C.sub.C51 and the base of the transistor Q52 and at the same time, the combination of a base resistor R72 and a constant current sink 72 sinking a constant current I.sub.O72 is provided to the base of the transistor Q53.
The base of the transistor Q52 is connected to the power supply line of V.sub.CC through the base resistor R71 and connected to the ground through the constant current sink 71. The corresponding terminal of the coupling capacitor C.sub.C51 is connected to the connection point of the base resistor R71 and the constant current sink 71. The base of the transistor Q53 is connected to the power supply line of V.sub.CC through the base resistor R72 and connected to the ground through the constant current sink 72.
A further example of the conventional oscillator circuits is shown in FIG. 4. This circuit 75' has the same configuration as that of the circuit 75 shown in FIG. 3 except that the crystal oscillator 51 is replaced with the crystal oscillator 51' shown in FIG. 2.
With the conventional oscillator circuits 55, 55', 75, and 75' shown in FIGS. 1 to 4, the coupling capacitor C.sub.C51 is used to interconnect the first-stage crystal oscillator 51 with the second-stage differential amplifier 53. When any one of the conventional oscillator circuits 55, 55', 75, and 75' is formed on an IC except for the crystal X.sub.tal and its relating capacitors C.sub.51 and C.sub.52, the coupling capacitor C.sub.C51 has a disadvantage that it occupies a large chip area of the IC. This disadvantage causes a first problem that the chip size and consequently the cost of the IC are increased.
Also, when any one of the conventional oscillator circuits 55, 55', 75, and 75' except for the crystal X.sub.tal and its relating capacitors C.sub.51 and C.sub.52 is subjected to various inspection tests to inspect the circuit operation, the oscillator circuit 55, 55', 75, or 75' needs to be operated at the practical operating frequency. Therefore, if the practical operating frequency is high, there arises a possibility that the tests cannot be carried out by using an existing IC tester or the like. This is due to the fact that the coupling capacitor C.sub.C51 tends to induce bad effects into the tests at such high operating frequency.
To cope with this case, a proper high-frequency generator is required to be provided. However, this creates a second problem that the procedures of the inspection tests are increased and the cost of these tests are raised.
Moreover, since the coupling capacitor C.sub.C51 tends to restrict the range of the operating frequency, there is a third problem that the applicable frequency is restricted. This third problem will limit the oscillation signal of the oscillator 51 or 51' or an external clock signal applied to the common-emitter amplifier 51a or 51a' through the input terminals T51 and T52.